The present invention relates to a high-speed web-fed flexographic printer, in particular, comprising at least one printing roller supporting a matrix strip, and an inking unit including an inking roller contacting the matrix strip on the printing roller.
Printers of the aforementioned type are generally known to feature a relief matrix in the form of a sheet of flexible polymer material, and may form part of high-speed printing plants, mainly for printing black and white publications. On book printing plants, the matrix strip is generally in the form of an endless strip wound on to the printing roller, containing all the matrixes relative to the pages in the book, and so varying in length accordingly.
For good quality printing, calendered or glazed paper of suitable substance must be employed; and known plants normally feature oil-based inking units comprising a series of rollers for grinding, mixing, spreading and transferring the ink on to the matrix strip.
Inking units of the aforementioned type present several drawbacks. To begin with, they are extremely cumbersome and complex as regards operation, due to the large number of rollers required for spreading the ink satisfactorily. The rollers are normally made of steel with a smooth outer surface to which the ink adheres mainly due to its viscosity, for spreading a thin film of ink on to the matrix strip.
As oil-based ink is absorbed very slowly by the above types of paper, a drier must be provided for drying the web as soon as it is printed, which drying process seriously dehydrates the paper, often to the extent of altering its physical characteristics and even its size, so that further provision must be made for rehydrating the web.
The high viscosity of oil-based ink also results in leftover ink accumulating to the side of the edge of the relief characters on the matrix, which leftover ink subsequently results in printed characters with a marked edge (edge effect) surrounding less heavily inked portions corresponding to the actual edge of the character, and by which the printed character is deformed and thickened.
Moreover, oil-based inking units do not permit multicolour printing, due to the difficulty in rapidly cleaning or changing all the rollers when switching from one color to another. Finally, oil-based inks also pose environmental problems, due to the nonbiodegradable and highly pollutant nature of the components involved, so that high-cost processes are required for disposing of leftover ink.
High-speed printers using water-based ink, e.g. for printing newspapers, have already been proposed. These, however, feature a flexographic matrix fitted to the printing roller and therefore of fixed length, so that they are unsuitable for printing books with a variable number of pages requiring a matrix strip of variable length.
What is more, printers of the above type normally employ low-cost paper of maximum 40 gr/sq.m substance, by which water-based ink is absorbed rapidly, thus resulting in printed characters with fairly uneven edges, and are therefore unsuitable for quality printing of calendered or glazed paper of a higher substance.